Coherent modulation is characterized by the fact that the frequency of the carrier is a multiple of the clock frequency of the binary signal to be transmitted. For high digital data rates, coherence is difficult to retain and so modulation is of a non-coherent type: phase skips take place at instants when the phase of the carrier has a random value .phi. uniformly distributed over the interval (0, 2.pi.). If the signal to be transmitted is represented by a successive of 0s and 1s each of duration T, and the signal is modulated with direct encoding using two phase states, correspondence is established between binary values 0 and 1 and respective selected ones of the two phase states. In order to facilitate recognition on demodulation, it is clear that the phase states should be as far as apart as possible: i.e. 0 and .pi..